The present invention relates to a moisture sensor comprising a thin layer of tantalum oxide applied to a moisture insensitive substrate and at least two electrodes placed on the tantalum oxide layer spaced apart from each other, and to a method of making such a moisture sensor.
DE-OS No. 2,728,092 describes a moisture sensor which comprises a base electrode consisting of a piece of tantalum, a tantalum oxide layer formed by anodic oxidation of the tantalum piece and an outer electrode covering a portion of the tantalum oxide layer. By the use of a moisture sensitive layer of tantalum oxide on a base electrode comprising a piece of tantalum the intention is to utilise the advantageous properties of tantalum and tantalum oxide for humidity and moisture sensors and to avoid the disadvantages of earlier known moisture sensors with metal oxide layer.
The most widely used metal oxide moisture sensors comprise an aluminium oxide layer on a base electrode of aluminium. Such moisture sensors have a very good sensitivity because the aluminium oxide is very porous and therefore absorbs moisture in an amount depending on the vapour pressure of the water in the ambient atmosphere, the capacitance and the resistivity of the aluminium oxide layer varying relatively greatly in dependence upon the moisture absorbed. However, moisture sensors on an aluminium basis have an inadequate electrical long time stability, are sensitive to mechanical and chemical impurities and are not suitable for use in corrosive or oxidising surroundings. On the other hand, tantalum and tantalum oxide are very resistant to corrosion and also have other favourable properties such as good long time stability, temperature stability and mechanical robustness so that a moisture sensor on a tantalum basis should be superior to a moisture sensor on an aluminium basis.
However, practice has shown that a moisture sensor of the type known from DE-OS No. 2,728,092 having a tantalum oxide layer formed by anodic oxidation of a piece of tantalum cannot readily replace a moisture sensor on an aluminium basis. For the tantalum oxide obtained in this manner is not very porous and thus only slightly moisture sensitive. Thus, a direct use of such a moisture sensor is possible at the most only in limited areas for special cases. On the other hand, to provide a generally usable moisture sensor the tantalum oxide must be subjected to a subsequent treatment by which it is given the necessary porosity and moisture sensitivity, for example by ion implantation as already known for aluminium oxide. This requires complicated apparatus.
The aforementioned advantageous properties of tantalum and tantalum oxide (Ta.sub.2 O.sub.5) have led to these materials being extensively used in the thin film technique wherein instead of bulk tantalum thin tantalum layers are formed by dust or vapour deposition on an insulating substrate (e.g. of glass or ceramic). If tantalum oxide layers are required they can be obtained by oxidation of previously applied tantalum layers. The tantalum layers may serve to form resistors and the tantalum oxide layers to form capacitor dielectrics. A distinction is made between two modifications of the thin tantalum layers formed by dust or vapour deposition which do not differ appreciably from bulk tantalum as regards their density and resistivity, i.e. an .alpha.-modification and a .beta.-modification. Compared with bulk tantalum the following values apply for these modifications:
______________________________________ Density Resistivity g/cm.sup.3 .mu. .OMEGA. .multidot. cm ______________________________________ Bulk tantalum 16.6 13 Thin layer .alpha.-modification 15.6 25-50 Thin layer .beta.-modification 15.9 180-220 ______________________________________
Due to the substantially same density of the metal the oxides of the .alpha.-modification and of the .beta.modification also do not differ appreciably from the oxide of the bulk tantalum. In particular, these oxides are also not very porous and thus not very moisture sensitive. The low moisture sensitivity is precisely one of the reasons why the oxides of the .alpha. and .beta.-modifications are preferably employed as capacitor dielectrics in thin layer capacitors or in integrated thin film circuits.
If however it were desired to use the oxide of the .alpha. or .beta.-modification of thin tantalum layers in a moisture sensor, it would have to be subjected to a similar subsequent treatment to the oxide of the bulk tantalum. The process steps necessary for this purpose would be incompatible with the usual thin film technique and could also not be carried out in existing apparatus for the production of thin film components.
The objective of the present invention is to provide a moisture sensor which has all the advantages of the moisture sensors on a tantalum basis but as regards sensitivity and measuring range is comparable to the moisture sensors on an aluminium basis and can be made with relatively simple process steps compatible with the conventional thin film technique.
Proceeding from a moisture sensor of the type set forth at the beginning this objective is achieved according to the invention in that the tantalum oxide film consists of the oxide of the highly resistive tantalum of low density.
The moisture sensor according to the invention is based on the use of the oxide of a special modification of tantalum which is also only present in thin films but which differs substantially from the previously mentioned .alpha. and .beta.-modifications and also from bulk tantalum as regards structure, density and resistivity. This modification is described in the following English-language publications where it is referred to as "low density" tantalum:
1. "Handbook of Thin Film Technology" by L. I. Maissel and R. Glang, McGraw Hill, 1970, pages 18-12 to 18-15;
2. "Thin Film Technology" by R. W. Berry and Van Nostrand Reinhold, N.Y., 1978 pages 226 to 231.
As apparent from these publications the low density tantalum is formed under certain conditions in thin layers in the sputtering of tantalum, and the sputtering voltage, i.e. the accelerating voltage of the argon ions and the partial pressure of argon, have been found to be essential parameters. With decreasing sputtering voltage the density of the tantalum in the deposited thin layer drops to values which are below 14 g/cm.sup.3 and can be as low as almost 10 g/cm.sup.3 whilst the resistivity rises to values which are already 5,000.mu..OMEGA.cm and more; values of up to 40,000.mu..OMEGA.cm can be achieved.
Because of these two properties distinguishing the "low density tantalum" from the .alpha. and .beta.-modifications and from bulk tantalum, this material will be called here "highly resistive tantalum of low density".
The present invention is based on the recognition that the oxide of the highly resistive tantalum of low density as regards mechanical strength, chemical resistance and electrical long time stability has similar favourable properties to those of ordinary tantalum oxide but differs from the latter in a porous structure rendering it particularly moisture sensitive. Thus, a moisture sensor can be obtained directly by oxidising a thin layer of highly resistive tantalum of low density, for example by anodic oxidation. The tantalum oxide layer thus obtained is suitable, without further structural change, as moisture sensitive layer and has a moisture sensitivity comparable with that of aluminium oxide. Both the application of a thin layer of highly resistive low density tantalum and the oxidation of such a layer are method steps which are compatible with the conventional thin film technique; however, the parameters must be correspondingly modified. All the other method steps for obtaining a moisture sensor according to the invention, such as shaping and attachment of the electrodes, can be made in accordance with the conventional thin film technique in existing apparatus for this purpose.
This compatibility affords the further possibility of forming on the same substrate simultaneously with the moisture sensor also the necessary evaluating circuit in integrated thin film technique. This makes the production simpler and more economical, gives a compact structure and because of the short electrical connections also gives better properties. By using the method known from DE-PS No. 2,714,034 for setting the temperature coefficient of thin film circuits with tantalum and/or tantalum oxide layers it is then even possible to give the entire integrated arrangement consisting of the moisture sensor and the evaluating circuit a definite temperature dependence.
A preferred method of making a moisture sensor according to the present invention resides in that by cathode sputtering or vaporisation a layer of highly resistive low density tantalum is deposited on a moisture insensitive substrate and that the tantalum layer is converted by oxidation over at least a portion of its thickness into a tantalum oxide layer.
It is however to be emphasised that the oxide of the highly resistive low density tantalum need not necessarily be made indirectly via a thin tantalum layer. It is also possible to apply a layer of the same porous oxide also directly to the substrate, either by carrying out oxidation during sputtering or vapour deposition from a tantalum source or by depositing tantalum oxide directly by sputtering or vapour deposition. The only requirement is that in both cases the parameters must be set so that the tantalum oxide layer has the same structure as a tantalum oxide layer formed by oxidation of highly resistive low density tantalum.
The moisture sensor according to the invention may be formed with all electrode structures already known in moisture sensors with aluminium oxide layers or other moisture sensitive materials, i.e. in particular with an outer electrode in the form of a thin continuous moisture permeable metal film or also with comb electrodes lying in a plane.
A particularly advantageous embodiment of the moisture sensor according to the invention is however distinguished by a novel outer electrode with lattice structure. The metal film of the lattice electrode need not be moisture permeable and can therefore be substantially thicker and more stable. It has also been found that the effect decisive for the moisture measuring takes place mainly along the edges of the electrode which in a lattice electrode may have a very great length per unit area.
Another advantageous embodiment of the moisture sensor according to the invention comprises comb electrodes made in particular manner.
Other objects and advantages of the present invention will be apparent from the following specification and claims together with the drawings.